Consolidated laser alignment and test station

ABSTRACT

A consolidated laser alignment and test station. In exemplary embodiments, equipment sufficient to perform complete dynamic testing and alignment of a laser transceiver unit is provided in one compact arrangement. As a result, cavity-box efficiency testing, dynamic open-interferometer alignment, dynamic open-case alignment, closed-case laser boresighting, and complete laser functionality and diagnostic testing can be carried out efficiently at a single location. Real-time diagnostic feedback relating to beam quality, radiometry, and temporal behavior is provided so that high-precision laser alignments and repairs can be made quickly and cost effectively. Customized test fixtures provide easy access to every level of the transceiver unit under test, and two cameras provide far-field, near-field, wide-field and receiver-field beam viewing. One camera is combined with a pin-hole lens and a quad step-filter optic attenuator to provide a wide-field beamfinder assembly enabling an operator to quickly align the laser under test to the narrower field of the second (diagnostic) camera. The second camera provides near-field and far-field beam viewing, while a radiometer and a pulse detector provide additional diagnostic information. The beamfinder assembly also provides receiver-field laser viewing for receiver-path boresight adjustments. In an exemplary embodiment, the beamfinder assembly includes a quad step-filter constructed from circular wedge filters.

This application is a divisional, of application Ser. No. 08/931,289,filed Sep. 16 1997, now U.S. Pat. No. 5,872,626.

BACKGROUND OF THE INVENTION

The present invention relates to laser transceivers, and moreparticularly to methods and apparatus for testing, aligning, andrefurbishing laser transceivers.

Today, laser radar (LADAR) and other systems incorporating lasertransmit-and-receive devices are in widespread use. For example, lasertransceivers are routinely employed in military applications for suchpurposes as target detection, acquisition, identification and tracking.However, due to the inherent high-precision nature of laser systems,testing and alignment of laser transceiver units can be difficult.Indeed, except for the simple adjustment of transceiver mounting screwsor the wholesale replacement of peripheral receiver assemblies, failedlaser transceivers are conventionally returned to the transceiversupplier for repair and maintenance. The supplier can provide both thefacilities and the skill required to carry out high-precision laseralignment and testing.

However, because the delays associated with shipping a laser transceiverunit to and from an appropriate supplier can be quite long, and becauselaser system suppliers typically have significant repair backlogs,conventional laser transceiver repair and maintenance is extremelycostly in terms of both time and money. As a result, laser system usersoften keep a large number of laser transceiver spares on hand. The costof doing so, however, can be prohibitive. Thus, there is a need forimproved methods and apparatus for testing, aligning and refurbishinglaser transceivers. In particular, there is a need for methods andapparatus allowing a relatively unskilled technician, who can be locatednearer the system user, to perform advanced laser maintenance andrefurbishment. Such methods and apparatus will yield significantimprovements in terms of laser repair cost and turnaround time and willgreatly enhance user self-sufficiency.

SUMMARY OF THE INVENTION

The present invention fulfills the above-described and other needs byproviding a consolidated laser alignment and test station which enablesa technician having relatively little specialized training to quicklyperform advanced laser repair and maintenance procedures. The teststation utilizes recent developments in electro-optic technologies toput all of the resources necessary for dynamically testing and aligninga laser transceiver unit within arm's reach of a single operator. Thus,the test station provides a high level of system repair through-put atsignificantly reduced cost.

In exemplary embodiments, equipment sufficient to dynamically test andalign NdYAG laser transceiver units is provided on a single compactbench. Thus; according to the present invention, cavity-box efficiencytesting, dynamic open-interferometer alignment, dynamic open-casealignment, closed-case laser boresighting, and complete laserfunctionality and diagnostic testing can be carried out at a singlestation. Real-time user feedback is provided so that high-precisionlaser alignments and repairs can be made quickly and cost effectively.

According to the present invention, a customized test fixture is used tomount the cavity box and interfrometer of a laser transceiver unit undertest to the test station bench. Thus, the cavity box and interferometerare mounted external to the laser transceiver unit case so thatfull-access laser-optics adjustments can be made. Additionally, avirtual laser bed fixture can be mounted in place of the lasertransceiver unit interferometer to permit direct testing of the cavitybox assembly itself. Furthermore, the entire closed-case lasertransceiver unit can be mounted on the test station bench for finaladjustments and boresighting of the laser transmitter and receiver.Generally, laser transceiver unit laser-optics adjustments are madewhile critical laser functions are simultaneously monitored via discretetest station readouts relating to beam quality, radiometry, and temporalbehavior.

In exemplary embodiments, two cameras provide far-field, near-field,wide-field and receiver-field beam viewing. The novel two-cameracombination allows a test station operator to efficiently view, measure,analyze and adjust a laser transceiver unit at all levels of operation.One of the cameras is combined with a pin-hole lens and a quadstep-filter optic attenuator to form a wide-field beamfinder assembly.Using the beamfinder assembly in conjunction with self-containedalignment optics included with the text fixtures described above, a teststation operator can quickly align the laser to the second camera. Thesecond camera then provides both near-field and far-field beam viewing,while a radiometer and a pulse detector provide additional diagnostics.In exemplary embodiments, the beamfinder assembly also providesreceiver-field laser viewing to allow for rapid boresight adjustments.

Advantageously, the modular and highly flexible design of the teststation allows it to be rapidly reconfigured to align and test a widevariety of laser transceiver units. For example, with appropriate opticinterface fixtures and suitable cameras, the test station canaccommodate NdYAG lasers operating at 1.064 μm, 0.532 μm, or 1.54 μm(Raman or wavelength shifted 1.064 μm) wave-lengths. Additionally, dualwavelength lasers (i.e., 1.5/1.064 μm) can be aligned and tested usingan appropriate collimator mirror and bolt-in dual-camera modules.Radiometry and image processing components of the test stationautomatically reconfigure to the laser transceiver unit under test.Because virtually any dynamic adjustment of a laser transceiver unitunder test can be performed at a single test station, laser testing andalignment can be conducted in a quick and cost-effective manner.Additionally, because the test station diagnostics providestraightforward real-time feedback, laser repair and maintenance can beachieved by a relatively inexperienced technician.

The above described and additional features of the present invention areexplained in greater detail hereinafter with reference to theillustrative examples shown in the accompanying drawings. Those skilledin the art will appreciate that the described embodiments are providedfor purposes of illustration and understanding and that numerousequivalent embodiments are contemplated herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a high-level block diagram of an exemplary interferometer ofthe type found within laser transceiver units that can be tested andaligned using the methods and apparatus of the present invention.

FIG. 2 is a conceptual diagram of an exemplary laser repair facility,incorporating a consolidated laser alignment and test station,constructed in accordance with one embodiment of the present invention.

FIG. 3 is a conceptual diagram of an exemplary test station constructedin accordance with the teachings of the present invention.

FIG. 4 is a block diagram of the basic components of an exemplary teststation constructed in accordance with the teachings of the presentinvention.

FIG. 5 depicts an exemplary reference plate, constructed in accordancewith the teachings of the present invention, which may be used to obtaina boresight reference for laser transceiver alignment.

FIG. 6 is a conceptual diagram of a reference laser beam following afar-field folded collimator path within a light-tight box of anexemplary test station constructed in accordance with the teachings ofthe present invention.

FIG. 7 is a conceptual diagram of a test laser beam following afar-field folded collimator path within a light-tight box of anexemplary test station constructed in accordance with the teachings ofthe present invention.

FIGS. 8(A), 8(B) and 8(C) are front, side and bottom views,respectively, of an exemplary test station constructed in accordancewith the teachings of the present invention.

FIGS. 9(A), 9(B) and 9(C) are top, side and front views, respectively,of an exemplary interferometer test fixture constructed in accordancewith the teachings of the present invention.

FIGS. 10(A), 10(B), 10(C) and 10(D) are side cross-section, front,bottom and perspective views, respectively, of an exemplary cavity-boxtest fixture constructed in accordance with the teachings of the presentinvention.

FIGS. 11(A), 11(B) and 11(C) are front, side and perspective views,respectively, of an exemplary beamfinder assembly constructed inaccordance with the teachings of the present invention.

FIG. 12 is a diagram of an exemplary method for constructing a step-wiseadjustable beam attenuator as taught by the present invention.

FIG. 13 is a diagram of a second exemplary method for constructing astep-wise adjustable beam attenuator as taught by the present invention.

FIGS. 14(A), 14(B), 14(C) and 14(D) are left-side, front, right-side andbottom views, respectively, of an exemplary four-step variable beamattenuator constructed in accordance with the teachings of the presentinvention.

FIG. 15 depicts an exemplary beamfinder assembly constructed inaccordance with the teachings of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A typical laser transceiver unit comprises three major subsystemsincluding a) a laser electronics unit, b) a laser interferometer and c)a cooling system. Generally speaking, the laser electronics unitprovides power-supply and timing-control signals to the interferometerwhich in turn establishes the pulse shape, pulse width, beam spread andspectral content of the beam transmitted by the laser transceiver unit.The cooling system dissipates heat which is generated within the lasertransceiver unit, for example by the power supply or by a triggeredlight source within the interferometer. The basic layout and operatingcharacteristics of a laser interferometer are described below. Laserelectronics units and cooling units are well known in the art and adetailed description of such systems is not necessary for anunderstanding of the present invention.

FIG. 1 is a high level block diagram of an interferometer 100 of thetype found in laser transceiver units which may be tested and alignedaccording to the teachings of the present invention. As shown, theexemplary interferometer 100 includes a switch 110 for controlling aPockels cell 120, a first terminating mirror 115, a first wave plate140, a cavity box 155 (including a flashlamp 125 and a laser rod 130), abeam splitter 145, a pair of lenses 150, a second wave plate 135, acorner reflector 160 and a second terminating mirror 185. In practice,each component of the interferometer 100 is mounted on a rigid base (notshown) to form a self-contained interferometer sub-assembly which can bemounted within a laser transceiver unit.

In FIG. 1, a first interferometer input 165 (labeled Trigger Pulse inthe figure) is coupled to a first input of the Pockels cell switch 110and to an input of the cavity box 155. Additionally, a secondinterferometer input 170 (labeled High Voltage Pulse) is coupled to aninput of the flashlamp 125, and a third interferometer input 175(labeled DC Supply Voltage) is coupled to a second input of the Pockelscell switch 110. An output of the cavity box 155 serves as aninterferometer output 180 (labeled Cavity Box Temperature). As shown,the first terminating mirror 115, the first wave plate 140, the laserrod 130, the beam splitter 145 and the pair of lenses 150 are arrangedto form a first optical path 190. At the same time, the firstterminating mirror 115, the first wave plate 140, the laser rod 130, thebeam splitter 145, the corner reflector 160, the Pockels cell 120, thesecond wave plate 135 and the second terminating mirror 185 are arrangedto form a second optical path 195.

In operation, a pulse forming network (not shown) is used to convert arelatively low-current, high-voltage (on the order of 1 kV) supply to amuch higher current pulse which is used to supply the flashlamp 125 viathe second interferometer input 170. At the same time, a triggergenerator (not shown) is used to control the flashlamp 125 via the firstinterferometer input 165 so that the flashlamp 125 emits triggeredpulses of non-coherent radiation which in turn stimulate pulsed laseremissions within the laser rod 130. A portion of the emitted laser beamfollows the first optical path 190 and serves as the laser output forthe laser transceiver unit in which the interferometer 100 is included.Another portion of the resulting laser beam follows the second opticalpath 195 and causes the laser to resonate (as the beam is reflected backand forth between the terminating mirrors 115, 185), and to therebymaximize lasing efficiency, as is well known in the art. Theinterferometer output 180 provides a measure of cavity box temperature,for example to control a cooling unit.

Generally, the wave plates 140,135 are used to control polarization ofthe internal beam, and the pair of lenses 150 is used to collimate andfocus the output beam. Additionally, a pair of wedged lenses or Risleys(not shown), which are typically mounted inside the laser transceiverunit casing and external to the interferometer itself, are used tosub-tune the direction of the laser output. The terminating mirrors 115,185 allow the second optical path 195 to resonate, as noted above, andthereby intensify the resulting laser output. The optional Pockels cell120 provides Q-switching for the resonator if it is desired. In otherwords, the controllable birefringence of the Pockels cell 120 is set toblock the second optical path 195 and to thereby prevent the laser fromresonating so that maximum energy is stored in the laser rod 130 priorto excitation of a laser pulse. Switching of the Pockels cell 120 iscoordinated with the triggering of the flashlamp 125 via the Pockelscell switch 110. As shown, the Pockels cell switch 110 receives theflashlamp trigger signal via the first interferometer input 165 as wellas a DC supply voltage via the third interferometer input 175.

According to the present invention, a consolidated laser alignment andtest station is used to assess and adjust the optics components of lasertransceiver units containing interferometers such as that depicted inFIG. 1. Those skilled in the art will appreciate, however, that theexemplary interferometer 100 of FIG. 1 is provided merely to aidexplanation of the present invention and that the test station of thepresent invention can be used to test and align a wide range of lasertransceiver units containing various interferometer configurations.Additionally, though the detailed description below makes reference toparticular physical embodiments of components of the present inventionwhich are tailored to accommodate a particular type of laser transceiverunit containing a particular type of interferometer, those skilled inthe art will recognize that equivalent embodiments can be constructed toaccommodate any laser transceiver/interferometer combination ofinterest.

FIG. 2 provides an overhead view of an exemplary laser repair facility200 incorporating a test station 210 such as that taught by the presentinvention. In the exemplary laser repair facility 200, the test station210 is situated proximate other laser transceiver unit repair equipmentso that a laser transceiver unit (including the interferometer, laserelectronics unit, cooling unit, outer chassis, etc.) can be fully testedand refurbished as necessary. As noted above, it may be advantageous tolocate a laser repair facility close to the ultimate laser transceiverunit user, for example near troops in the field for militaryapplications. Thus, the laser repair facility 200 of FIG. 2 may beconstructed to fit within, for example, a mobile trailer.

As shown, the exemplary laser repair facility 200 comprises three roomsor work areas including first and second clean rooms 205, 215 and acooling unit refurbishment area 225. The first clean room 205 includesan exemplary test station 210, and the second clean room 215 includes atear-down bench 220 and a static alignment bench 230. Generally, thefirst clean-room 205 is used to dynamically test and align the opticsportions of the laser transceiver unit, and the second clean room 215 isused for laser transceiver unit tear-down, optics inspection, opticscleaning and bonding, and static alignment of the interferometer. Thecooling unit refurbishment area 225 is used to decontaminate andrefurbish the laser transceiver unit cavity box and cooling systemsub-assemblies as necessary.

In operation, an incoming laser transceiver unit is first tested on thetest station 210 while it is operating to obtain data on the lasertransceiver unit performance and to perform a preliminary diagnosis ofany detected failures. Following confirmation of a laser failure, thelaser transceiver unit is moved to the tear-down bench 220 where thecavity box and cooling unit assemblies are removed and taken to thecooling unit refurbishment area 225. There the cooling unit and cavitybox assemblies are tested and refurbished as necessary (e.g.,replacement of faulty cavity box reflectors, cleaning and replacement ofthe laser rod, replacement of liquid coolant, etc.). Preferably at thesame time, the interferometer optics are cleaned and any unserviceableoptics components are replaced at the tear-down bench 220. Thereafter,the interferometer is statically realigned at the static alignment bench230, and laser electronics unit repairs and any upgrades or retrofitincorporations are performed as necessary. Once cleaning, refurbishmentand static alignment are complete, the laser transceiver unitsub-assemblies are re-integrated, dynamically aligned, boresighted, andfinal-tested using the test station 210.

In exemplary embodiments, a depot repair form is generated (e.g., withina laser repair facility computerized data base) when a laser transceiverunit first enters the laser repair facility 200. The depot repair formis then continually updated throughout the laser transceiver unit repairprocess to document the repair and maintenance history of the particularlaser transceiver unit. If appropriate, the completed depot repair formis downloaded to a central computerized laser transceiver unit trackingsystem (not shown). Those skilled in the art will appreciate that thebrief description of the exemplary laser repair facility 200 providedabove is intended to further understanding of the present invention andthat the beneficial aspects of the test station embodiments described indetail below are equally applicable to any form of laser repairfacility.

FIG. 3 is a conceptual diagram showing the basic components of theexemplary test station 210. As shown, the exemplary test station 210comprises a hood 305 and a test bench 370. The test bench 370 includes atest platform 390 and a light-tight beam housing 380. In FIG. 3, a laserelectronics unit 310, a laser run box 320, a waveform display unit 330,a display monitor 340 and a radiometer display unit 350 are positionedon the bench 370. Additionally, a laser transceiver unit under test 360is mounted on the test platform 390.

Generally, the laser electronics unit 310 and the laser run box 320 areused to provide power and control signals to the laser transceiver unit360 just as they would be provided to the laser transceiver unit 360during operation in the field. The laser transceiver unit 360, or asub-assembly of the laser transceiver unit 360 (e.g., theinterferometer), is fired through apertures in the testing platform 390and the top portion of the test bench 370 and into the beam housing 380.As described below, laser measurement equipment attached to the beamhousing 380 is used to provide a test station operator with real-timediagnostic information relating to the beam emitted by the lasertransceiver unit 360. The diagnostic information is provided to the teststation operator via the waveform display unit 330, the display monitor340 and the radiometer display unit 350 as described below. Using thereal-time diagnostic feedback, the test station operator can quicklytest and align the laser transceiver unit 360 (and the laser transceiverunit sub-assemblies) as necessary.

FIG. 4 is a high-level schematic diagram showing electric and opticinterconnections between components of the exemplary test station 210.As shown, the test station 210 includes the laser electronics unit 310,a pulse detector 430, the light-tight beam housing 380, an adjustableoptical attenuator 450, a near/far field camera 460, a beamfinder camera499, a beam processor 470, a laser source 498, the display monitor 340,a detector head 475, an attenuator controller 455, a radiometer 465, adata bus 445, a computer interface 485 and an input keyboard 495. Asshown, the laser electronics unit 310 is bi-directionally coupled to afirst input of the computer interface 485 and also bi-directionallycoupled to an input of the transceiver unit under test 360. An opticaloutput of the laser transceiver unit 360 is coupled to a first input ofthe beam housing 380.

First, second and third outputs of the beam housing 380 are opticallycoupled to inputs of the pulse detector 430, the detector head 475, andthe camera 460 respectively. An electric output of the camera 460 iscoupled to an input of the beam processor 470, and the beamfinder camera499 is selectively positionable between the first camera 460 and thebeam processor 470. An electric output of the attenuator controller 455is coupled to an input of the adjustable optic attenuator 450, and anoptical output of the laser source 498 is coupled to a second opticalinput of the beam housing 380 via a fiber optic cable 497. An electricoutput of the detector head 475 is coupled to an input of the radiometer465, and an electric output of the pulse detector 430 is coupled to asecond input of the computer interface 485. An output of the computerinterface is coupled to an input of the display monitor 340, and anoutput of the keyboard 495 is coupled to a third input of the computerinterface 485. The beam processor 470, the attenuator controller 455,the radiometer 465 and the computer interface 485 are all coupled viathe data bus 445 which may be, for example, an IEEE 488 bus.

As described above, optical energy from the transceiver unit under test360 is fired into the light-tight beam housing 380 and real-timediagnostic feedback is provided so that the test station operator canefficiently perform testing and alignment as necessary. The beam housing380 contains two folded collimator paths which are integrated with(i.e., aligned with) the variable attenuator 450, the camera 460, thepulse detector 430, the detector head 475, the radiometer 465, and thelaser source 498 (which may provide a boresight reference source and areceiver range testing source as described below). Generally, the camera460 and the beam processor 470 provide detailed beam viewing, forexample via the display monitor 340. Both near-field and far-field viewscan be displayed by switching between the two folded collimator pathswithin the beam housing 380 as described below.

The detector head 475 and the radiometer 465 provide beam energydiagnostics (e.g., via the radiometer display unit 350), and the pulsedetector 430 provides laser pulse waveform diagnostics (e.g., via thewaveform display unit 330 which may comprise an oscilloscope). Thevariable attenuator 450 is used to set saturation levels for the camera460. The computer interface 485 provides central control of the teststation components, user prompting at appropriate points in the test andalignment process, and detailed data, processing and management.

In exemplary embodiments, the digital image processor 470 providesreal-time far-field divergence optimization and measurement over a rangeof 250 μR to 2.0 mR, radiation outside the main beam, near-field beamviewing including profile top-hat and isometric, and boresight alignmentand measurement. Additionally, the radiometer 465 output provides energyper pulse with 2% accuracy, average power from 20 mW to 30 W, missingpulses, pulse frequency and time jitter, pulse width, and secondarypulses.

As described in detail below, the present invention teaches thatcustomized fixtures can be utilized to allow the test station operatorto test the laser transceiver unit 360 at every level of laseroperation. In other words, in addition to mounting the entire lasertransceiver unit 360 to the test platform 390, the test station operatorcan use specialized test fixtures to mount sub-assemblies to the testplatform 390 so that they may be tested independently of other lasertransceiver unit components. For example, the present invention providesan interferometer test fixture whereby an interferometer which has beenremoved from its laser transceiver unit housing can be mounted on thetest platform 390 for open-case testing and alignment. Additionally, thepresent invention provides a cavity box test fixture whereby a cavitybox which has been removed from its interferometer assembly can bemounted on the test platform 390 and tested directly.

Advantageously, in addition to providing for diagnostics with respect tobeams emitted from the laser transceiver unit, the test station 210 alsoprovides for boresight referencing and receiver testing as describedbelow. Throughout the testing and alignment process, the test stationoperator utilizes a novel beamfinder assembly, also described in detailbelow, to quickly align the various beams to the diagnostic camera 460.A more detailed explanation of the various aspects of the exemplary teststation 210 is next provided.

In exemplary embodiments, a boresight reference source (e.g., a laserdiode generating a HeNe-type laser output 635 nm) is used to achieveprecision laser boresight alignment. Advantageously, the HeNe-typereference beam is integrated with and afocal to the far-field collimatorpath, thereby providing a collimated HeNe-type output which isautocollimated from a novel reference fixture mounted on the testplatform 390. To boresight a transceiver unit under test, the teststation operator mounts the reference fixture on the test platform 390in place of the unit under test. As described below, the referencefixture includes a partially reflecting reference plate and a reflectingcorner cube which provide two reflections of an impinging laser beam.The test station operator then fires the boresight reference source intothe beam housing 380 such that a beam emitted by the reference sourceexits a test beam entry port of the beam housing and reflects back fromthe reference fixture to provide two reference beam spots on thediagnostic image provided by the diagnostic camera 460.

As described in more detail below, the test station operator can thenadjust the position of the boresight reference source so that the tworeference beam spots are aligned (e.g., on the display monitor 340).Next, the test station operator records the position of the alignedreference beam spots, removes the reference fixture from the testingplatform 390, mounts the laser transceiver unit under test in its place,and fires the laser transceiver unit into the beam housing. By viewingthe laser transceiver unit beam spot via the diagnostic camera 460, thetest station operator can precisely boresight the laser transceiver unitby adjusting the direction of the laser transceiver unit beam (e.g., byusing a pair of Risley's included in the laser transceiver unit) untilthe laser transceiver unit beam is positioned at the recorded referencebeam spot position.

FIG. 5 depicts an exemplary reference fixture 500 constructed inaccordance with the teachings of the present invention. As shown, theexemplary reference assembly 500 comprises a reference plate 530including a front reflective surface 540, a reflective corner cube 520,and a protective housing 510. The reference fixture 500 provides tworeflections 550, 560. The first 560 is from the front surface 540 of thereference plate 530, which provides the laser mounting plane reference.The second 550 is from the corner cube 520, which defines the boresightreference axis. As described above, the two reflections 550, 560 projectback through the collimator within the beam housing 380 to thediagnostic camera 460. While viewing the camera's monitor 340 andcontrolling a motorized gimbal connected to the boresight referencesource (as described below), the boresight reference axis is aligned tocoincide with the laser mounting plane reflection 560, and the digitalimage (beam) processor 470 stores that location in memory for laseralignment referencing as described above.

FIG. 6 is a conceptual top view of a reference laser beam following afar-field folded collimator path within a light-tight beam box 625 whichis included in the beam housing 380 of the exemplary test station 210 asdescribed below. As shown, an exemplary reference configuration 600comprises the laser source 498, the fiber optic cable 497, a fiber opticattenuator/collimator 645, the camera 460, the radiometer 465, agimballed mirror 640, the light-tight beam box 625, two beam splitters650,680 and five folding mirrors 655, 660, 665, 670, 675. The lasersource 498 includes a receiver range testing source 697, a boresightreference source 698 and an optical combiner 699. The receiver rangetesting source 697 can be, for example, a 1.064 μm source, and theboresight reference source can be, for example, a 635 nm wavelengthdiode laser.

In the figure, a reference beam 615 emitted by the laser source 498passes through the fiber optic cable 497, the fiber optic collimator 645and the beam splitter 650. The reference beam 615 may be a boresightreference generated by the 635 nm source 698 or a receiver range testingreference generated by the 1.064 μm source 698, depending on whichtesting function is being performed. The test operator powers up eitherthe 1.064 μm source 697 or the 635 nm source 698 as appropriate, and thecombiner 699 couples the resulting reference beam 615 to the fiber opticcable 497.

The reference beam 615 is deflected by the folding mirrors 655, 665, 675and beam splitter 680 to the gimballed mirror 640 where it is deflectedto the reference fixture 500 mounted on the testing platform 390 (e.g.,the gimballed mirror 640 deflects the beam out of the page in FIG. 6 sothat it passes through an aperture in the top of the test bench 370 andtoward the test platform 390). As shown, the beam(s) reflected back fromthe reference fixture 500 deflect off of the gimballed mirror 640, thebeam splitter 680, the folding mirrors 675,665,655 and the beam splitter650 to the diagnostic camera 460 as desired. The adjustable attenuator450 is used to control the saturation level of the camera 460.

FIG. 7 is a conceptual top view of a test laser beam 705 following afar-field folded collimator path within the light-tight box 625 includedin the beam housing 380 of the exemplary test station 210. As shown, anexemplary test configuration 700 comprises the camera 460, theradiometer 465, the gimballed mirror 640, the light-tight beam box 625,the two beam splitters 650,680 and the five folding mirrors 655, 660,665, 670, 675. Such a configuration would be used, for example, duringthe latter stages of the boresighting procedure or during generaldiagnostic assessment of a laser transceiver unit and itssub-assemblies.

In the figure, a test beam 705 emitted from a laser transceiver unit orlaser transceiver unit sub-assembly mounted to the test platform 390(e.g., passing through an aperture in the top of the test bench 370)deflects off of the gimballed mirror 640, the beam splitter 680, thefolding mirrors 675,665,655 and the beam splitter 650 to the diagnosticcamera 460 as desired. As described above, the camera 460 provides animage to the digital beam processor 470 which in turn provides real-timebeam diagnostics to the test station operator via the display monitor340. As before, the adjustable attenuator 450 is used to control thesaturation level of the camera 460.

Additionally, a portion of the beam 705 passes through the beam splitter680 and impinges on the detector head (not shown) of the radiometer 465to provide radiometry information to the test station operator. Theadditional folding mirrors 660,670 are used to selectively provide anear-field collimator path as described below.

FIGS. 8(A), 8(B) and 8(C) show front, side and bottom views,respectively, of the exemplary test station 210. As shown, the exemplarytest station 210 comprises the hood 305, the test bench 370, a cavitybox test fixture 805, an interferometer test fixture 810, the testplatform 390. a beamfinder assembly 815 (housing the beam findercamera499) and the gimballed mirror 640. The test station 210 also comprises afirst sub-housing 820, a second sub-housing 830, a light tube 835 andthe beam box 625, which collectively form the light-tight beam housing380. As shown in FIG. 8(C), the test station 210 also includes first andsecond corner reflectors 840, 845, the camera 460, a camera cable 850,the variable attenuator 450, a field selector 860, the fiber opticattenuator 645, and the radiometer 465.

During testing and alignment, the test station operator mounts a lasertransceiver unit under test, or the reference fixture 500, directly tothe test platform 390. Alternatively, the test station operator can usethe interferometer test fixture 810 to mount an interferometer standingalone. Additionally, the cavity box test fixture 805 can be used incombination with the interferometer test fixture 810 to mount a cavitybox standing alone, for example for cavity box efficiency testing. Thetest laser transceiver unit, the test interferometer, or the test cavitybox is then fired through an aperture in the test platform 390, throughan aperture in the bench top and into the beam housing 380. Within thebeam housing 380, the test beam is deflected from the gimballed mirror640, through the light tube 835, off the mirrors 840,845 and into thebeam box 625. Within the beam box 625, the test beam is directed to thevarious diagnostic components as described above. In exemplaryembodiments, the beam box 625 is constructed as described above withreference to FIG. 6. Such a beam box is available from Coherent, Inc. ofSanta Clara, Calif.

As described in more detail below, the beamfinder assembly 815 of FIGS.8(A) and 8(B) is used to align the test beam to the diagnostic camera460. In other words, because the field of view of the diagnostic camera460 is narrow (tailored to provide diagnostics for a pin-point beam),the relatively wide field-of-view beamfinder assembly 815 is used sothat the test station operator can quickly position the beam within theviewing range of the diagnostic camera 460. When a device under test isfirst fired into the beam housing 380, the beamfinder assembly 815 ispositioned between the device under test and the top of the test bench370. The test station operator then views an image of the test beamwhich is provided by the wide-field beamfinder camera 499 within thebeamfinder assembly (e.g., via the display monitor 340). Next, theoperator makes coarse adjustments (e.g., using the mirrors or theRisleys provided on the device under test) to bring the test beam withinthe field of view of the diagnostic camera 460 (e.g., by matching thebeam spot provided by the beamfinder camera 499 to a cross-hair providedon the display monitor 340). Finally, the operator removes thebeamfinder assembly 815 and proceeds with laser testing and alignmentusing the primary diagnostic camera 460.

FIGS. 9(A), 9(B) and 9(C) show top, side and front views, respectively,of the exemplary interferometer test fixture 810. As shown, theexemplary interferometer test fixture 810 comprises a cavity boxmounting plate 920 (including screw holes 995), mounting bolts 905,interferometer mounting points 935, an electrical connector 930, anelectrical cable 915, a pair of rotatable wedged lenses or Risleys 910,and a beam aperture 925. During testing and alignment, an interferometerunder test is mounted to the mounting plate 920 via the screw holes 995.The test fixture 810 is then mounted to the testing platform 390 via themounting bolts 905. The electrical cable 915 and the connector 930 areused to provide power supply and control signals to the interferometerunder test. During testing, the interferometer fires through the beamaperture 925 which corresponds to a beam aperture in the mountingplatform 390. The Risleys 910 simulate those found within the lasertransceiver unit from which the interferometer was removed and are usedto adjust the direction of the beam.

FIG. 10(A) is a cross-sectional side view of the exemplary cavity-boxtest fixture 805. Additionally, FIGS. 10(B), 10(C) and 10(D) show front,bottom and perspective views, respectively, of the exemplary cavity-boxtest fixture 805 shown in FIG. 10(A). The exemplary cavity box testfixture 805 includes two terminating mirror housings 1030, fourterminating mirror control knobs 1005, a body 1045, a vertical mirrorhousing 1025, a first handle 1020, a second handle 1010, a directionalmirror 1035, and two directional mirror control knobs 1015.

During testing and alignment, a laser cavity box is mounted to the body1045 of the cavity box test fixture 805, the cavity box test fixture 805is mounted to the interferometer test fixture 810, and theinterferometer test fixture 810 is mounted to the test platform 390. Thecavity box is then fired, and a reflecting mirror within the verticalmirror housing 1025 directs the emitted test beam up and back to thedirectional mirror 1035. The directional mirror 1035 then directs thebeam through the test aperture 925 of the interferometer test fixture810 and toward the beam housing 380 for diagnostic measurement.Terminating mirrors within the terminating mirror housings 1030 are usedto simulate those of the interferometer from which the cavity box undertest was removed. The terminating mirror control knobs 1005 and thedirectional mirror control knobs 1015 are used to adjust the terminatingmirrors and the directional mirror 1035, respectively.

FIGS. 11(A), 11(B) and 11(C) show front, side and perspective views,respectively, of the exemplary beamfinder assembly 815. As shown, theexemplary beamfinder assembly 815 comprises a base 1105, mounting screws1110, a quad-step adjustable attenuator 1115, a filter housing 1120, anda camera housing 1125. During testing and alignment, the beamfinderassembly 815 is positioned beneath the testing platform 390 to providefor quick coarse adjustments as described above. The adjustableattenuator 1115 is used to set saturation levels for the wide-fieldbeamfinder camera 499 which is contained within the camera housing 1125.In exemplary embodiments, the adjustable attenuator 1115 is astep-filter providing four levels of attenuation. Such a step-filter canbe constructed as described in detail below. The beamfinder camera 499of the beamfinder assembly 815 is coupled to the display monitor 340 ina fashion which is well known in the art.

In exemplary embodiments of the test station 210, a receiver testingsource (e.g., the 1.06 μm NdYAG laser source 697) generating an outputsimulating that of a laser transceiver unit under test is used to fire abeam through the test station beam housing and into a receiver assemblyassociated with the laser transceiver unit under test. The receiver testequipment includes the receiver testing source 697, the laser attenuator645, an EMI screen box (not shown), and a receiver interface adapter(not shown). The laser receiver is mounted in an EMI resistant metal boxto reduce the possibility of false alarms due to other electronicequipment. Access holes in the box allow probing of test points andsetting of trim pots for sensitivity adjustments. The receiver ismounted on the testing platform 390 via the interface adapter, whichprovides all electrical connections necessary for power and I/O signals.An optical port on the EMI box allows laser stimulus to enter thereceiver's optic aperture. The optical port is held by machinetolerances to provide a boresighted input.

A noted above, the optical signal for receiver (or boresight)sensitivity testing and adjustment can be provided by a fiber opticcoupled laser diode source operating at the 1.06 μM wavelength. Suchdevices are commercially available and are controllable via an IEEE-488bus. The precise output of the optical signal is determined byattenuating the pulse generated by the optical source with the fiberoptic attenuator 645. The fiber optic attenuator 645 can control thesignal over 50 dB with a resolution of 1%. The output of the fiber opticattenuator 645 is sent via the fiber optic cable 497 to a collimatinglens that underfills the receiver's optical aperture. The underfillingof the aperture insures that all the energy from the fiber opticattenuater 645 is sent through the receiver's field stop eliminatingradiometric errors due to aperture tolerances and speckle effects. Inalternative embodiments, a coiled fiber optic bundle can be attached tothe beam box in place of the fiber optic attenuator for range testing.

As noted above, the beamfinder assembly 815 may use a multiple-stepattenuator to set saturation levels for the beamfinder camera 499. Whileadjustable filters are available commercially, they tend to be somewhatbulky and relatively expensive. Advantageously, the present inventionteaches that a useful step-wise adjustable attenuator can be constructedinexpensively using commonly available circular wedged filters. Inexemplary embodiments, the wedged filters are inexpensive 1″ diametercircular filters having an unknown degree of wedge. As is well known inthe art, the wedge tends to deflect a beam passing through the filterand is not a desirable filter feature in the context of the presentinvention. Advantageously, the present invention teaches that astep-filter having substantially no wedge effect can be constructedusing the inexpensive and imprecise circular filters.

FIG. 12 depicts an exemplary method for constructing a variable beamattenuator according to the present invention. As shown, the exemplarymethod comprises eight steps S1-S8. In step S1, a first circular wedgedfilter 1200 is marked with a bisecting line 1202. In step S2, the wedgedfilter 1200 is cut along the bisecting line 1202 to create a leftportion 1205 and a right portion 1210. In step S3, the right portion1210 is rotated 180° in a plane defined by an upper surface of the rightportion 1210 and positioned beneath the left portion 1205. In step S4,the left portion 1205 and the right portion 1210 are bonded to form afirst half-filter 1215 providing a first level of optical attenuation.Because the wedge effects caused by the left and right portions willlargely cancel each other, the first half-filter 1215 will includesubstantially no wedge effect.

In step S5, steps S1 through S4 are repeated using a second wedgedfilter to create a second half-filter 1220 providing a second level ofoptical attenuation and including substantially no wedge effect. In stepS6, the first half-filter 1215 and the second half-filter 1220 arebonded edge-wise to form a first two-step optical attenuator 1225. Instep S7, steps S1 through S6 are repeated using third and fourth wedgedfilters to form a second two-step filter 1235. In step S8, firsttwo-step filter 1225 and second two-step filter 1235 are bonded to forma quad step-filter 1240. Thus, the present invention teaches that afour-step variable attenuator can be constructed inexpensively usingfour circular wedge filters. As described below with reference to FIGS.13 and 14, the eight pieces (i.e., the two halves of each of the fourcircular filters) can be configured in an alternative fashion to providea four-step filter having very low wedge.

FIG. 13 depicts an exemplary method for constructing a half-filter suchas that described above with reference to FIG. 12. As shown, in step S1Atwo marks 1310 are scribed on the edge of the circular filter 1200 atopposite ends of an imaginary arbitrary center line 1315 crossing theface of the filter 1200. At step S2A, the circular filter 1200 is cutalong a bisecting centerline which is offset from the edge marks by 0.1inches to form the left piece 1205 and the right piece 1210. At stepS3A, the right piece 1210 is rotated 180 degrees in the plane of FIG.13. Finally, at step S4A, the right piece 1210 is positioned beneath theleft piece 1205 so that the scribe marks 1310 are aligned. Because somematerial is removed during the cutting process, the two pieces will beslightly offset from one another as shown. Rather than bonding the facesof the left and right pieces directly to one another as described abovewith respect to FIG. 12, the two pieces can be configured with sixcorresponding pieces cut from three additional circular wedge filters toprovide a superior quad step-filter.

FIGS. 14(A), 14(B), 14(C) and 14(D) show left side, front, right sideand bottom views, respectively, of such an exemplary four-step filter.As shown, the quad-step filter comprises four pairs 1410, 1420, 1430,1440 of semicircular filter pieces, each pair processed in accordancewith the method of FIG. 13. In FIG. 14, the eight pieces are matched incorresponding pairs and interleaved to form a four-layer quad filterhaving four sections each providing a distinct level of opticattenuation with substantially no wedge effect. As shown, the pieces ineach layer are alternately offset from a centerpoint of the quad filter,and adhesive is applied at the resulting recesses 1405 so that the quadfilter is substantially cylindrical and mechanically sturdy.

FIG. 15 depicts the exemplary beamfinder assembly 815 employing astep-filter such as that shown in FIG. 14. As shown, the beamfinderassembly 815 comprises the quad step-filter 1115, a pin hole aperture1520, the camera housing 1125 and an adjustment knob 1525. A center axis1505 of the quad filter 1115 is positioned such that a center portion ofone quadrant of the step-filter 1115 is aligned with a line of sight1510 defined by the pin-hole aperture 1520 and a camera within thecamera housing 1125. During testing and alignment, the test stationoperator positions the beamfinder assembly underneath the testingplatform 390 as described above. The operator then selects a desiredlevel of filter attenuation (i.e., a desired camera saturation level) byrotating the step-filter 1115 until an appropriate section of the filter1115 is aligned with the camera. The operator then tightens theadjustment knob 1525 to secure the filter 1115 in place.

As the foregoing discussion makes clear, the present invention teachesmethods and apparatus which significantly improve the art of laserrepair and maintenance. By placing extensive dynamic testing andalignment resources within arm's reach of a single operator, the presentinvention enables a technician having relatively little specializedtraining to obtain a high level of system repair through-put atsignificantly reduced cost. Empirical studies have proven that thesynergies created by embodiments of the present invention yield laserrepair success rates, in terms of both speed and accuracy, heretoforeunheard of in the art of laser repair and maintenance.

Those skilled in the art will appreciate that the present invention isnot limited to the specific exemplary embodiments which have beendescribed herein for purposes of illustration. The scope of theinvention, therefore, is defined by the claims which are appendedhereto, rather than the foregoing description, and all equivalents whichare consistent with the meaning of the claims are intended to beembraced therein.

What is claimed is:
 1. A method for making a step-wise variable optic attenuator, comprising the steps of: a) cutting a first wedged filter in half to create a first filter piece and a second filter piece; b) rotating the second filter piece 180 degrees relative to the first filter piece; c) bonding the rotated second filter piece to the first filter piece to provide a first half-filter having a first level of optic attenuation, wherein wedge effects caused by the first filter piece are largely canceled by wedge effects caused by the second filter piece so that the first half-filter has substantially no wedge effect; d) repeating said steps of cutting, rotating and bonding using a second wedged filter to provide a second half-filter having a second level of optic attenuation and substantially no wedge effect; and e) bonding the first half-filter and the second half-filter to provide a step-wise variable optic attenuator having first and second levels of optic attenuation and substantially no wedge effect.
 2. The method of claim 1, comprising the additional steps of: repeating said steps a) through e) using third and fourth wedged filters to provide a second step-wise variable optic attenuator having third and fourth levels of optic attenuation and substantially no wedge effect; and bonding the first step-wise variable optic attenuator and the second step-wise variable optic attenuator to provide a quad optic attenuator having four levels of optic attenuation and substantially no wedge effect.
 3. A step-wise variable optical attenuator, comprising: a first half-filter providing a first level of optic attenuation and including first and second filter pieces, wherein said first and second filter pieces are cut from a first single wedged filter, and wherein said first and second filter pieces are arranged such that wedge effects caused by the first filter piece substantially cancel wedge effects caused by the second filter piece so that the first half-filter causes substantially no wedge effect; and a second half-filter providing a second level of optic attenuation and including third and fourth filter pieces, wherein said third and fourth filter pieces are cut from a second single wedged filter, and wherein said third and fourth filter pieces are arranged such that wedge effects caused by the third filter piece substantially cancel wedge effects caused by the fourth filter piece so that the second half-filter causes substantially no wedge effect.
 4. The step-wise variable optical attenuator of claim 3, further comprising: a third half-filter providing a third level of optic attenuation and including fifth and sixth filter pieces, wherein said fifth and sixth filter pieces are cut from a third single wedged filter, and wherein said fifth and sixth filter pieces are arranged such that wedge effects caused by the fifth filter piece substantially cancel wedge effects caused by the sixth filter piece so that the third half-filter causes substantially no wedge effect; and a fourth half-filter providing a fourth level of optic attenuation and including seventh and eighth filter pieces, wherein said seventh and eighth filter pieces are cut from a fourth single wedged filter, and wherein said seventh and eighth filter pieces are arranged such that wedge effects caused by the seventh filter piece substantially cancel wedge effects caused by the eighth filter piece so that the fourth half-filter causes substantially no wedge effect, wherein said first, second, third and fourth half-filters are arranged to provide four filter sections each providing a distinct level of optic attenuation and each causing substantially no wedge effect. 